1. Field of the Invention
The present invention relates to a method and apparatus for measuring blood information based on an image signal of a blood vessel.
2. Description Related to the Prior Art
Endoscopes are widely used for observing lesions in subjects. The endoscope is provided with an insert section to be inserted into the subject and a handling section for operating the insert section. A distal end of the insert section is formed with a lighting window and an imaging window. An internal body portion is imaged under illumination. An observation image obtained is displayed on a monitor.
Generally, a white light source such as a xenon lamp or a metal halide lamp is used as a light source for the endoscope. Recently, it has become popular to use narrowband illumination light, which facilitates finding a lesion (see U.S. Patent Application Publication No. 2008/0281154 corresponding to Japanese Patent No. 3583731).
A method for measuring blood information, e.g. an oxygen saturation level or a blood flow rate, of blood in a blood vessel has been researched (see Japanese Patent Laid-Open Publication No. 06-315477). In this method, the blood vessel is extracted from an endoscopic image captured under illumination of narrowband light. The blood information is measured based on an image signal of the blood vessel. The method utilizes the narrowband illumination light in wavelength bands of 300 to 400 nm, around 400 nm, 400 to 500 nm, 500 to 600 nm, and 450 to 850 nm. For example, one of the above five wavelength bands most suitable for measuring the oxygen saturation level of hemoglobin is selected in accordance with the internal body portion. In the wavelength band selected, two different types of narrowband light are used as a wavelength set. One of the two types is measurement narrowband light that has a wavelength at which absorbance substantially varies with the oxygen saturation level. The other is reference narrowband light that has a wavelength at which the absorbance is unaffected. The two types of narrowband light are applied successively to the internal body portion. An image signal obtained from the measurement narrowband light is corrected using an image signal obtained from the reference narrowband light. Thereby, the oxygen saturation level of the blood in the blood vessel is obtained.
Depth of penetration of light into human tissue differs depending on the wavelength band. The wavelength sets of the illumination light are switched to measure the oxygen saturation level of blood in a blood vessel in each of the layers, from a mucosal layer to a deep layer. Thereby, a depth of invasion of a cancer lesion is detected.
The Japanese Patent Laid-Open Publication No. 06-315477 does not specifically disclose switching timing of the wavelength set. When the wavelength set is switched manually, a complicated operation is required, which is time-consuming. The internal body portion may move during the switching operation. On the other hand, when the wavelength set is switched automatically, the switching may take place during the observation of normal tissue. This results in unnecessary calculation of the oxygen saturation.